1. Field of the Invention
The present invention relates to a corrosion-resistant overlay to be applied to the edge of a valve disk and in particular to a method for applying the overlay in such manner as to assure proper positioning of the overlay and its retention in place during operation of the valve.
2. Description of the Prior Art
The prior art has recognized the need for an overlay on the edge portion of a disk in a valve with the overlay assuring proper seating of the disk in the valve. However, the prior attempts to provide such an overlay have resulted in a number of problems. For example, it is difficult to get effective sealing of elastomeric material between the disk body material and a corrosion-resistant overlay because of the creation of sealant voids, unattained critical tolerancing or leakage through the elastomeric seal itself. Other problems include ineffective joining of the ends of the corrosion-resistant overlay, due to the thin cross section and lack of weld penetration to prevent burning of the underlying seal. Another problem is the lack of positive seal ring constraints to prevent the ring from damaging downstream equipment should the primary weld fail allowing the overlay to move downstream. A further problem is the lack of positive means to prevent rotation of the overlay about the disk, such rotation generally placing the welded joint in the area of highest turbulence thereby increasing the danger of the joint breaking. Yet another problem is the critical diametrical tolerancing of a stainless steel ring overlay which, if too long, causes excessive clearance between the disk edge and the body resulting in seal leakage or an oversize disk diameter and, if too short, causes excessive weld gap resulting in a weak joint or a higher probability of seal burn during welding.
Examples of the prior art can be found in U.S. Pat. No. 3,749,359 which teaches a sealing ring made of an endless member. This ring is applied by roll forming to create a compressive metal-to-metal axial grip of the ring on the valve disk, best illustrated in FIG. 7.
Another example of the prior art may be found in U.S. Pat. No. 3,958,314 in which a strip overlay is placed on a lip of either the valve seat or valve member, with the ends of the strip in abutting relationship and welded together. This reference includes a number of schemes for applying a seal between the overlay and the valve disk as illustrated in FIGS. 3-7. The embodiment of FIG. 5, which is disclosed at column 3, lines 33-47, has bores at one or more locations on the valve disk communicating with a peripheral groove. A flowable, setable plastic sealant, such as epoxy resin, is introduced through the port to flow about the periphery of the disk to seal the inner face. There is no provision for assuring that the peripheral groove will be entirely filled and that voids will be avoided.
U.S. Pat. No. 4,141,537 has a valve seat with a frustoconical seating surface. The overlay has a cross-sectional thickness progressively decreasing from its smaller end to its larger end to provide flexibility. The valve seat is described only in terms of a cylindrical ring member.
The present invention has for its primary object the provision of a corrosion-resistant overlay on an edge portion of a valve disk or valve member and in particular to the application of the overlay in such manner as to prevent the above-discussed disadvantages from occurring.